In modern digital and analog chips, digital signals are transmitted via one or more signal lines coupling a transmitting circuitry to a receiving circuitry. For transmitting digital “0” states and “1” states via the signal line, specific voltage levels are effected on the signal line. The specific voltage level caused on the signal line depends on the output impedance of the transmitting circuitry and on the current on the signal line. At the side of the receiving circuitry, a “0” state or a “1” state is detected by comparing the voltage on the signal line to a receiver reference voltage.
Normally, the characteristic impedance of the signal line should be matched to the output resistance of a driver in the transmitting circuitry and a termination resistance in the receiving circuitry in order to avoid parasitic signal reflections. Thus, the amplitude of a data eye obtained at the receiving circuitry can be appropriately positioned with respect to the reference voltage.
The above considerations are of importance in off-chip drivers (OCDs) and, in particular, push-pull off-chip drivers, which are widely used in modern digital and analog chips to drive digital signals onto a signal bus coupling a transmitting circuitry to a receiving circuitry. Generally, the OCD should provide for a certain impedance in order to obtain a matched termination of the printed circuit board transmission lines, or coaxial cables, connecting the OCD to a receiving circuitry so that requirements for a desired signal swing on the transmission lines can be fulfilled.
What is needed are means for providing an adjusted voltage drop across a circuit such that a desired signal swing on a signal line connected to an output node can be obtained.